1. Field of the Invention
The present invention relates to a gradation record control apparatus for an ink jet printer which can control the gradation of an image to be recorded.
2. Description of the Related Art
In recent years, ink jet printers have come to be used widely as output printers for domestic and office computers from a viewpoint that the ink jet printers are quiet during a recording period, can achieve a high-speed recording and can easily print in colors. These ink jet printers make the ink to be flown in droplets and carry out a recording by adhering the flown droplets onto a recording sheet. The ink jet printers are broadly classified into a continuous system and an on-demand system according to a method of generating droplets and a method of controlling a flying direction.
The continuous system is a system as disclosed, for example, in U.S. Pat. No. 3,060,429, according to which ink droplets are generated by an electrostatic absorbing, the generated droplets are electrolytically controlled according to a recording signal and the droplets are selectively adhered onto the recording sheet to thereby carry out a recording. This system requires a high voltage for generating droplets and has a difficulty in having a multi-nozzle. Thus, the continuous system is not suitable for a high-speed recording.
The on-demand system is a system as disclosed, for example, in U.S. Pat. No. 3,747,120, according to which an electrical recording signal is added to a piezo-oscillating element which is provided in a recording head having a nozzle aperture for jetting droplets. This electrical recording signal is changed to a mechanical oscillation of the piezo-oscillating element and the droplets are jetted from the nozzle aperture according to the mechanical oscillation and are adhered onto the recording sheet to thereby carry out a recording. Since the ink is jetted from the nozzle aperture on demand to carry out a recording, unlike the continuous system, it is not necessary to recover the droplets which have not been used for recording an image after the ink droplets were flown. Therefore, the on-demand system can take a simpler structure. However, the on-demand system has drawbacks in that it is difficult to process the recording head, that it is difficult to have multi-nozzles because of an extreme difficulty in achieving a compact piezo-oscillating element, and that the system is not suitable for a high-speed recording since the droplets are flown by a mechanical energy which is a mechanical oscillation of the piezo-oscillation element.
Further, as another type on-demand system, there is a recording system for flying liquid drops by boiling the ink with a heat generated by a heat generating resistor, as disclosed in JP-B-61-59911, JP-B-62-11035 and JP-B-61-59914, respectively.
Further, there is still another on-demand system which utilizes a thermal energy, instead of utilizing a mechanical oscillation energy by means of the piezo-oscillation element or the like, as disclosed in U.S. Pat. No. 3,179,042. The system which utilizes a thermal energy has characteristics that the energy conversion efficiency is high and that it is easy to have multi-nozzles, as compared with the system which utilizes the mechanical oscillation energy.
The principle of ink jetting according to the above-described system which utilizes a thermal energy will be explained below. FIG. 7 is a structural diagram for showing a conventional gradation record control apparatus. In FIG. 7, 37 denotes a conductive ink, 38 an ink chamber filled with the conductive ink 37, 39 an ink tank for accommodating the conductive ink 37, and 40 and 41 a pair of electrodes disposed at a lower level than the liquid surface of the conductive ink 37. 42 denotes a power source for applying a voltage to the electrodes 40 and 41, 43 a switch for selectively applying a power to the electrodes 40 and 41, and 44 a nozzle for jetting the conductive ink 37. 45 denotes a recording sheet and 46 denotes an ink drop jetted from the nozzle 44. When a voltage is applied to the electrodes 40 and 41, a current flows to the conductive ink 37, and a part of the conductive ink 37 between the front ends of the electrodes 40 and 41 is vaporized by Joule heat of the current. The steam of the vaporized conductive ink 37 is expanded until a pressure which is sufficient enough to jet the ink drop 46 onto the recording sheet 45 from the nozzle 44 has been generated. By on-and-off control of the switch 43, a nozzle aperture for jetting the conductive ink 37 is selected and a desired character is formed on the recording sheet 45.
However, when the ink jetting device is to be used as a gradation record control apparatus, it is necessary to apply different voltages to the electrodes 40 and 41 in order to change the size of the ink drops 46. For this purpose, it is necessary to have switching elements and power sources for changing over different voltages corresponding to the number of gradations for each nozzle of a plurality of nozzles. For example, in order to change each of 100 nozzles in eight gradations, 800 switching elements and eight power sources are necessary, which has a problem that the cost of circuits becomes extremely high. Further, when a voltage is to be changed over for each scan by assuming that each row is scanned eight times, there is a problem that the printing speed is reduced to one eighth although the circuit scale becomes smaller.